1. Field of the Invention
The present invention concerns a method for injection molding a metal mold by melting low fusion point metal materials such as zinc, magnesium or their alloys.
2. Detailed Description of the Prior Art
Die casting is adopted for casting low fusion point nonferrous metal; however, die casting requires a smelter for melting metal material completely, and the casting is performed by pumping molten metal from this smelter, or by extruding by a plunger. Therefore, granular metal material supplied from the rear of a heating cylinder is being melt by transporting forward in the heating cylinder by screw rotation, accumulated and weighed in a heating cylinder antechamber, injected and charged into a die from a nozzle at the heating cylinder extremity by screw advancement, similarly as plastic material, without melting by a smelter.
Problems encountered in case of adopting such injection molding for metal material are difficulty of melting and transport of metal material by screw rotation and weighing instability.
Plastic material is melt mainly by shearing heat generation; therefore, the screw diameter is formed to increase as approaching the extremity section, and the screw groove forming the material flow gap is formed relatively shallow. However, molten plastic is transported smoothly forward by screw rotation even when the flow gap is formed narrow, because of the difference of friction factor at the interface surface of the heating cylinder inner wall.
On the other hand, the viscosity of metal material completely molten to the liquid phase state being incomparably lower than the plastic material, there is almost no difference of friction factor at said two interface surfaces, so the transport power by screw rotation as in case of molted plastic generates hardly. In addition, in a low viscosity liquid phase state, a pressure elevation so strong as pushing back the screw does not generate, and the screw retraction by the material pressure hardly occurs, and the accumulated amount in the antechamber differs only by the screw rotation, making technically difficult to quantify the injection filling quantity.
Therefore, it has been proposed to injection molding a metal material in semi-molten state by limiting the melting temperature to a temperature equal or superior to the solid phase line temperature and equal or inferior to the liquid phase line temperature, without melting the metal material completely. In said temperature range, the structure of molten metal is supposed to be in semi-molten state (thixothropic state), and in this state, the flow resistance generates in the molten metal, allowing to transport by the screw rotation and to weigh by its retraction. Consequently, the molding of metal molds by the conventional injection molding has been performed adopting such method.
However, such molding method is nothing but a straight application of plastic material injection molding means, and it is difficult to maintain the molten metal temperature in the heading cylinder, as the metal is high in the heat conductivity different from plastic material. Even in the metal material molding, the heating cylinder keeps the set temperature being heated by an outer periphery band heater. However, the screw side has no heating means, and moreover, heat is easily radiated from the rear end section. Consequently, the temperature of molten metal in the screw grooves becomes easily irregular and this can not be preventing by the agitation by screw rotation is assumed to be impossible, because the screw per se serves as molten metal material transport member, and provokes an excessive supply of material.
This invention devised to solve the aforementioned problems in case of injection molding of metal material in semi-molten state has an object to provide a novel injection molding method allowing to agitate molten metal in a melting vessel by combining separately functioning agitation means and injection means without adopting a screw having triple functions of melting, transport and injection supposed to be indispensable in the prior art, and to mold metal molds having a thixothropic structure in a state where the metal material melting temperature is kept within a set temperature range by this.
For the aforementioned object, this invention consists in supply continuously or discontinuously granular metal material in a cylindrical fusion vessel having a required length of weighing chamber communicating with a nozzle port at the extremity section, including rotatively an agitation means inside, and where an injection means whose extremity section is formed into an injection plunger is inserted advanceably and retractably into the center section of this agitation means, for engaging slidably said injection plunger into the weighing chamber, heat melting the metal material to a temperature equal or superior to the liquid phase line temperature by an external heat and, at the same time, accumulating in the fusion vessel by homogenizing by the aforementioned agitation means, injecting and filling a die by cooling to a temperature equal or superior to the solid phase line temperature and equal or inferior to the liquid phase line temperature in the course from the feed into the weighing chamber by the retraction of the aforementioned injection plunger to the injection and filling of the die, and forming into a metal mold having a thixothropic structure.
Also, this invention consists in supply continuously or discontinuously granular metal material in a cylindrical fusion vessel having a required length of weighing chamber communicating with a nozzle port at the extremity section, including rotatively an agitation means inside, and where an injection means whose extremity section is formed into an injection plunger is inserted retractably into the center section of this agitation means, for engaging slidably said injection plunger into the weighing chamber, heat melting the metal material to a temperature equal or superior to the solid phase line temperature and equal or inferior to the liquid phase line temperature by an external heat and, at the same time, accumulating in the fusion vessel by agitating and maintaining melt metal material in a semi-melt state (thixothropy state) by the aforementioned agitation means, sending a part of this semi-melt state metal material into the weighing chamber by the retraction of the aforementioned injection plunger, injecting and filling by advancing the injection plunger keeping the semi-melt state event after the weighing, and forming into a metal mold having a thixothropic structure.